Preheat oven to 400°F. Line a 9" pie dish with pie crust.
For pie filling:
In a mixing bowl, combine rhubarb, strawberries, egg, sugar, flour and vanilla. Stir gently until thoroughly combined.
Spoon into pie shell.
Combine flour and brown sugar in a mixing bowl.
Cut in slices of cold butter with a pastry blender until mixture is pea size crumbled.
Sprinkle topping mixture over top of pie.
Place pie on a cookie sheet (not necessary - but far easier to remove when done baking) and bake at 400° for 10 minutes.
Reduce heat to 350° and bake for an additional 25 - 30 minutes or until topping is golden brown.
Cool pie on wire rack.
Slice, serve and enjoy.
Store pie covered in fridge. Best eaten within a day or two of baking.